Sure, the Pilgrims are credited with starting the tradition of Thanksgiving in America.

But declaring Thanksgiving Day a formal holiday on the fourth Thursday in November required a pair of presidents -- two of the most famous, in fact.

Abraham Lincoln and Franklin D. Roosevelt.

It was Lincoln who issued an 1863 proclamation calling on Americans to "set apart and observe the last Thursday of November next as a day of thanksgiving," partly to celebrate victories in the then-raging Civil War.

"He's the father of the whole idea of a nation giving thanks for its advantages and privileges of living in a democracy like this," said Harold Holzer, historian and chairman of the Abraham Lincoln Bicentennial Foundation.

Decades later, Roosevelt and Congress acted to establish Thanksgiving on the fourth Thursday of November, in part to lengthen the Christmas shopping season.

Informally, the United States government had recognized periodic days of thanksgiving from the moment of the nation's birth.

In 1777, a year after the Declaration of Independence, the Continental Congress declared a day of thanksgiving to celebrate a Revolutionary War victory over the British at Saratoga.

The first president, George Washington, declared a day of thanksgiving and prayer in 1789, partly to honor the new U.S. Constitution.

It took the trauma of the Civil War to make Thanksgiving a formal, annual holiday.

Lincoln issued his proclamation on Oct. 3, 1863, three months after Union Army victories at Gettysburg and Vicksburg, and at a time in which ultimate triumph appeared in sight. "There was a lot to be thankful for in the fall of 1863," said Allen Guelzo, the Henry R. Luce Professor of the Civil War Era at Gettysburg College.

Writing that the nation's many blessings "should be solemnly, reverently, and gratefully acknowledged" by the American people, Lincoln declared: "I do therefore invite my fellow-citizens in every part of the United States, and also those who are at sea and those who are sojourning in foreign lands, to set apart and observe the last Thursday of November next as a day of thanksgiving and praise to our beneficent Father who dwelleth in the heavens."

(It should be noted that while Lincoln issued this proclamation, most historians believe it was actually written by his secretary of State, William Seward.)

The proclamation served a familiar purpose for Lincoln. "He was always looking for ways to unify the nation in a terrible time of war," biographer Ronald C. White Jr. said.

Still, the idea of a formal Thanksgiving holiday had been gestating for a long time before Lincoln.

Sarah Josepha Hale, editor of the popular magazine Godey's Lady's Book, pushed the idea for years, petitioning Lincoln and his predecessors. (Thanksgiving was only one of Hale's contributions to American culture; she also wrote the poem Mary Had a Little Lamb.)

After Lincoln, presidents issued annual Thanksgiving proclamations. In last year's proclamation, President Obama said Thanksgiving Day is "a time to take stock of the fortune we have known and the kindnesses we have shared, grateful for the God-given bounty that enriches our lives."

For decades after Lincoln, Americans traditionally celebrated Thanksgiving on the last Thursday of November, even if it fell on Nov. 30 -- as happened in 1939, the end of an economically troubled decade.

Some merchants worried that a late Thanksgiving would cut down on Christmas sales and asked President Roosevelt to move the holiday up by a week - which he did, creating unanticipated havoc.

Some state governors objected, issuing proclamations of their own to keep Thanksgiving on Nov. 30. Other states recognized the Nov. 23 date. This created scheduling issues for holiday traditions ranging from family reunions to football games.

"It was just chaos for a couple of years," said Bob Clark, supervisory archivist with the Franklin D. Roosevelt Presidential Library & Museum.

Congress eventually stepped in. On Dec. 26, 1941, less than a month after the attack at Pearl Harbor plunged the United States into World War II, Congress passed a law declaring the fourth Thursday of November as Thanksgiving Day, where it remains to this day.